Use a fixed-point format specifier inside the f-string’s braces: f"{value:.2f}" displays two digits after the decimal point. It adds trailing zeroes when needed, so 7.1 becomes 7.10. The result is formatted text; it does not change the original number.
Use .nf for a fixed number of decimal places
The general pattern is f"{number:.nf}", where n is the number of digits to show after the decimal point. The f selects fixed-point formatting, and the precision after the dot specifies the fractional digits.
number = 7.1
print(f"{number:.0f}") # 7
print(f"{number:.2f}") # 7.10
print(f"{number:.4f}") # 7.1000A precision of zero still uses fixed-point formatting, but displays no digits after the decimal point. For two places, for example, the official Python tutorial uses {math.pi:.3f} to display pi as 3.142. See the Python 3.14.7 tutorial on input and output.
Set the number of places dynamically
If the required precision is stored in a variable, put a nested replacement field in the format specification:
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value = 12.34567
places = 3
formatted = f"{value:.{places}f}"
print(formatted) # 12.346This uses the value of places as the precision. PEP 498 documents expressions in format specifications, including dynamic precision: PEP 498 – Literal String Interpolation.
Choose decimal places, not significant digits
Use .nf when the output must have exactly n digits after the decimal point. The .ng form instead specifies significant digits, and may switch to scientific notation depending on the value and precision.
| Format | What precision counts | Example output for 1234.567 |
|---|---|---|
.2f |
Digits after the decimal point | 1234.57 |
.2g |
Significant digits | 1.2e+03 |
For fixed decimal places, choose f. Use g when significant-digit precision—and notation that can adapt to magnitude—is what you need. Python’s format specification reference defines these precision rules.
Formatting rounds the display, not the stored value
Formatting creates a string representation; it does not overwrite the input variable. For floating-point fixed-point formatting, Python uses a rounding mode that matches the built-in round(), as documented in the built-in types reference.
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Also, a displayed value such as 12.35 does not prove that the underlying binary floating-point value is exact to two decimal places. For financial or other decimal-sensitive calculations, choose a suitable numeric representation and calculation-rounding policy; use the f-string specifier to control the final display. Decimal values can use the same f-string format-specification mechanism, as shown in PEP 498.
Format percentages without confusing display and scaling
To show a value that is already expressed in percentage points, format it and append the percent sign:
percentage = 12.5
print(f"{percentage:.2f}%") # 12.50%If the value is instead a fraction such as 0.125, decide whether it should be displayed as 0.13% or scaled to 12.50%. Scaling and formatting are different operations. The percent presentation type scales the value for percentage display; use it when that behavior is intended. The Python tutorial’s percentage example multiplies the value by 100 before formatting it with two decimal places.
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